# [Proj] OSTN02 question: GRS80 instead of Airy.

Frank Warmerdam warmerdam at pobox.com
Mon Oct 30 20:11:52 EST 2006

```Roger Oberholtzer wrote:
> Or not. If I change the +ellps=airy to +ellps=grs80, leaving all else
> the same, I see no difference at all.
>
> Ad an example, for lat=50.34180000 lng=-4.82595000, I get
> Easting=199020.37000 Northing=52918.58000 for both airy and grs80.
>
> It could be that these should be the same. But I thought grs80 would
> result in some small difference.

Roger,

I agree.  The conversion from geographic coordinates on the destination
ellipsoid to projected coordinates on the destination ellipsoid should
be different when the ellipsoid is different.  The shapes of Airy and
GRS80 are noticably different (by over 500m on the semi-major axis) so
I'd expect the difference to be noticable.

I'd suggest isolating this to small steps on the commandline.

When I isolate this one step via the proj command I get:

warmerda at gdal2200[198]% proj +proj=tmerc +lat_0=49 +lon_0=-2 +k=0.9996012717
+x_0=400000 +y_0=-100000 +ellps=airy  +units=m +no_defs
-4.82 50.34
199361.89       52768.58

warmerda at gdal2200[199]% proj +proj=tmerc +lat_0=49 +lon_0=-2 +k=0.9996012717
+x_0=400000 +y_0=-100000 +ellps=GRS80 +units=m +no_defs
-4.82 50.34
199342.42       52781.88

So yes, it seems you should see a difference on the order of 20 meters in
X and Y.

If I use cs2cs (which calls pj_transform()) for something close to your
case, bug using +towgs84 since I don't have gb.gsb, I also see differences
though in this case the transformation from ellipsoid to ellipsoid has
a substantial effect.

warmerda at gdal2200[202]% cs2cs +proj=latlong +datum=WGS84 +to +proj=tmerc
+lat_0=49 +lon_0=-2 +k=0.9996012717 +x_0=400000 +y_0=-100000 +ellps=airy
+units=m +towgs84=0,0,0 +no_defs
-4.82 50.34
199358.98       52691.74 527.32

warmerda at gdal2200[200]% cs2cs +proj=latlong +datum=WGS84 +to +proj=tmerc
+lat_0=49 +lon_0=-2 +k=0.9996012717 +x_0=400000 +y_0=-100000 +ellps=GRS80
+units=m +towgs84=0,0,0 +no_defs
-4.82 50.34
199342.42       52781.88 0.00

Are you sure you are having no effect?  Perhaps you could provide gb.gsb
for us to use for a test?

Best regards,
--
---------------------------------------+--------------------------------------
I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | President OSGeo, http://osgeo.org

```